Qwen 3.5 MoE Metal: Use max-sized prefill example for dynamic inputs

[manuelcandales]

With alloc_graph_input=False, ExecuTorch sets the input tensor's
numel_bound_ from the serialized example size. A small example (T=2)
prevents runtime inputs larger than 2 tokens. Use max_seq_len-1 as
the prefill example size so any prompt length is accepted at runtime.

Authored with Claude.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• -> Qwen 3.5 MoE Metal: Use max-sized prefill example for dynamic inputs #18956
• Qwen 3.5 MoE: Add Metal build target #18881
• Qwen 3.5 MoE: Add --backend metal export path #18880
• Qwen 3.5 MoE: Add Metal source transformations #18879
• Metal backend: Add gated delta rule kernel for linear attention #18878
• Metal backend: Add gather_qmv kernel for MoE expert-indexed quantized matmul #18877
• Metal backend: Add topk fallback kernel via MPSGraph #18876
• Metal backend: Add SDPA head_dim=256 support #18875
• Metal backend: Add INT32 and BOOL dtype support #18874

[pytorch-bot]

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